Xerographic transfer process



L. E. WALKUP XEROGRAPHIC TRANSFER PROCESS Aug. 12, 1958 Filed D60. 16, 1954 2 Sheets-Sheet 1 "A I A ulll llir- INVENTOR LEWIS EM/ALKUP I MAMM- ATTORNEY Aug. 12, 1958 E. WALKUP XEROGRAPHIC TRANSFER PROCESS 2 Sheets- Sheet 2 Filed Dec. 16, 1954 ATTO R N EY United States Patent 2,847,305 XEROGRAPHIC TRANSFER PROCESS Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc., Rochester, N. Y., a corporation'ofNew York v Application December 16, 1954, Serial No. 475,746

' 1 Claim. (Cl. 96-4) This invention relates to xerography and in particular to the'transfer of electroscopic powder images after development, especially under conditions of high relative humidity. v

In the art of xerography as disclosed in Carlson Patent No. 2,297,691, it is usual to place an electrostatic charge on a xerographic member which is generally composed of a photoconductive layer overlying a conductive backing. This charged member is then exposed to copy to be reproduced and the electrostatic charge on the photoconductive insulated layer is selectively discharged, thereby leaving an electrostatic latent image on the photoconductive surface. This latent image may then be developed by depositing thereon electroscopic particles carrying charges opposite to those composing the electrostatic latent image. These charged particles adhere to areas of charge on the insulating layer, thereby developing an electrostatic image pattern. This image pattern may be utilized as it exists on the surface of the insulating layer, or and preferably, it may be transferred to other material.

One of the usual methods of transfer is to cause particles of electroscopic material forming the developed image pattern to move from the insulating surface to another surface, for example, paper by the use of electrostatic forces. This transfer technique is not particularly diflicult in ordinary circumstances. In the past, however, a number ofditficulties have been encountered in employing the electrostatic transfer process of xerography at high relative'humidities. Such difiiculties are to be expected, because xerography depends upon minute charges of static electricity which can be drained away over the smallest leakage paths. The practical importance of the effect on transfer of humidity has become greater in recent years because of the increasing quality expected of xerographic products. It is common now to compare xerography with other commercial processes in terms of the results produced and it is expected to work almost perfectly all times and under all conditions.

It has been found that for practical purposes, using commonly known transfer procedure, it is difiicult if not impossible to transfersatisfactorily to paper or to offset duplicating mats when either is in equilibrium with air at high relative humidities. Objects and features of this invention are to provide improved techniques of transfer of developed xerographic images to paper or to offset duplicatingmats that yield effective and satisfactory results irrespective of the relative humidity conditions in existence at the time transfer is effected.

In its most common form electrostatic transfer comprises laying a sheet of paper over the developed toner image on the xerographic plate, delivering an electrostatic charge to the paper, usually by means of a corona discharge, and peeling the paper from the plate. This operation transfers to the paper approximately one-half of the image toner powder present on the plate. This procedure is satisfactory for most applications when humidity is low. However, this transfer fails when the paper is made slightly electrically conductiveas by the absorption of too much moisture, for example, under conditions of highrelative humidity. The reason for this failure with moist paper is that the electrical charge on the paper drains away so rapidly that the potential cannot build up to the usual 300 to 600 volts necessary for adequate transfer of powder image to the paper. While one way to improve transfer in such procedure would be to reduce the lateral electrical conductivity of the paper this cannot be done as a practical matter except by keeping it dry until actually used. However, this is difficult from a packaging standpoint and involves other objectionable features as well as added expense which make it preferable to provide other simple and inexpensive techniques for effecting the transfer irrespective of humidity conditions or of moisture content of the transfer paper or material.

While lateral conductivity in the moist paper is the principal cause of transfer failure at high relative humidities this lateral conductivity would have no deleterious effect on electrostatic transfer if there were no paths over which the electrical charge could leak from the paper to the ground. The elimination or else the material reduction of such leakage is proposed as a mode or technique for overcoming the difficulties of transfer to damp or moist paper under conditions of high relative humidity.

The leakage of the electrical charge from the moist transfer paper to ground affects the quality of the transfer because it reduces the potential on the paper. Leakage in itself is not otherwise objectionable unless the current is great enough to damage the electrostatic plate. If therefore the potential on the paper during transfer can be maintained long enough to transfer the toner image,

there would be no harm in some current leaking to ground during or after transfer.

Objects and features of this invention are the provision of an effective method of causing adequate transfer to moist transfer paper or other material during conditions of high relative humidity.

Other objects and features of the invention are the provision of methods for effecting transfer under such conditions that are simple and inexpensive.

Further objects and features of the invention will become apparent from the following specification and from the accompanying drawing in which:

Figure l is a diagrammatic view of an arrangement and method for eflecting transfer, for example, to paper under conditions of high relative humidity; and

Figure 2 is a diagrammatic view of a modified arrangement and method for practicing the invention.

Referring to the drawing the reference character 10 denotes a xerographic plate or element comprising a backing member 11 of conductive material such as aluminum provided on one surface with a photoconductive coating 12, for example, of amorphous selenium. As is known a plate of this character, for example as described in the aforesaid Carlson patent, is charged electrostatically and its surface 12 is then exposed to copy producing a latent electrostatic image thereon. This image is then developed by blowing or cascading electroscopic'powder over the surface of the coating 12. This powder adheres to the surface 12 in conformity with the residual charges of the image pattern producing a developed powder or toner image on the surface of the layer 12.

In effecting transfer of this toner image it has heretofore been a common practice to deposit a sheet of paper or offset mat material over the entire surface of the layer 12 hearing the powder or toner image and with the paper lying on said surface applying a charge to the paper of opposite polarity to that of the charge of the powder in the toner image thereby causing transfer of the powder or toner image from the surface 12 to the adjacent surface of the paper sheet or mat. This works as long as the paper sheet or mat is neither wet nor excessively moist as would be its condition under conditions of high relative humidity.

In order to provide effective transfer under conditions of the latter kind a transfer sheet 13 of paper or offset mat material onto which transfer is to be effected is not simply spread over the entire surface of the coating 12. Instead a grounded electrode 14 is clipped or otherwise connected to one of the ends of a sheet 13. The opposite ends of this sheet of paper are held by hand and the backing member 11 of the xerographic element is raised to a potential of approximately 6000 volts negative by connecting it to the negative electrode of a high D. C. voltage source 15. The two ends of the transfer sheet 13 are held by the hand so that the sheet has a curve or sag Whose lowermost portion 16 extends into contact with the toner image-bearing surface of the coating 12. The leading edge ,17 of the sheet is lowered progressively and simultaneously the lagging edge 18 of the sheet is lifted progressively by the two hands of the operator and in consequence the area of contact at 16 between the sheet 13 remains substantially the same as it rolls along the length of the coating 12 from the lagging to the leading edges of the sheet 13. At each instant there is only a small area 16 of the sheet 13 in physical contact with the image-bearing surface of coating 12 although ultimately the entire surface of the sheet 13 has had contact with the surface of coating 12 because of the progressive roll of contacting portion 16 therewith. In other words the area 16 of the transfer sheet 13 at any one time in contact with the coating 12 is comparatively limited and much smaller than the entire surface area of the sheet 13. Consequently since the same amount of current is being delivered at all times from the high voltage source during the transfer step, with a reduced contact area 16 at all times,. the charge densities in the contact area 16 will be greater and as a result provide an increase in the potential on the paper in the area 16 of contact. For example, if the contact area 16 is one-fiftieth of the total area of the sheet 13, the charge density distributed over area 16 for a specific potential is fifty times as large as the charge density that would exist for the same potential if the entire sheet 13 were contact with the coating 12 at one time. In consequence, the potential on the paper in the reduced area 16 of contact is similarly increased and is sufiicient to provide a sufficient potential difference between the transfer paper and the image surface in the area of contact to cause effective transfer of the powder toner image on the coating 12 to the sheet 13.

It is to be noted that the transfer effected by this roll-down technique is not simultaneous, instead a progressive transfer of the image on various portions of the coating 12 occurs as the area 16 of contact rolls progressively from one end to the other of the image-bearing coating 12. Moreover, after transfer occurs in any specific area of the sheet 13, that area is immediately separated from the coating 12.

Transfer as effected by the roll-down technique with reduced area of contact as just described and immediate separation after transfer eliminates substantially the difiiculty of image transfer to sheets of paper or offset mats under conditions of high relative humidity. A further advantage of this particular technique is that the curvature imparted to the sheets during rolling down smooths out any wrinkles, bulges or other irregularities in the sheet along the direction of the span of contact 16 between the sheet and the coated surface 12 immediately prior to and during transfer. This insures uniformity of contact between the sheet and the coating during the roll-down operations.

The roll-down technique as herein described is applicable as is illustrated diagrammatically in Figure 2 for use in presently existing xerographic copying devices. In usual transfer practice with such a device the xerographic plate 10a bearing a powder image on its coating 12a is inserted into the copier device 20 after having had a transfer sheet 13a deposited fiat over the image-bearing coating or surface 12a. In the copier, closure of a switch causes a corona discharging element 21 to move reciprocally over the surface of the entire transfer sheet 13 to charge the latter and effect electrostatic transfer to said sheet of the powder image. This is satisfactory procedure when low relative humidity prevails.

Under high relative humidity conditions, however, the roll-down technique of the instant invention is substituted for said procedure in the following Way. After the transfer sheet 13a has been deposited over the imagebearing surface coating 12a of the xerographic element 10 and the assembly has been inserted into the copier 20, the corona discharging device 21 is energized but is maintained in a stationary position adjacent the plate exit slot 22 of the copier. While maintaining the corona discharge device 21 stationary and in an energized condition the outer or leading edge of plate 10a isgrasped by one hand of the operator and slowly withdrawn from the copier 20 through the slot 22. At the same time the leading or outer edge of the transfer sheet 13a is grasped by the other hand of the operator and manipulated to curve and roll the transfer sheet 13a away from the plate 10a. In this latter roll-away operation a slight downward pressure is exerted on the curved sheet 13a to keep the transfer sheet 13a in firm contact at 23 with the image bearing coating 12a of the xerographic plate 10a. This produces cylindrical curvature at 13b in the transfer sheet which tends to flatten it in the area at 23 against the coating 12a of plate 10a, said area 23 being directly under the corona discharge device 21 where transfer is taking place. In addition, the sheet 13a is immediately stripped from its contact at 23 with coating 12a as transfer occurs and the area of contact between the transfer sheet 13a and the coating 12a of the xerographic element 10a is progressively reduced as the sheet 13a is separated from the coating 12a of plate 10a so that leakage paths to ground are reduced. Very effective transfer of powder toner images onxerographic plate elements under conditions of high relative humidity have been secured wih this technique.

As with the first described technique, the roll procedures of this invention maintains uniform charging potential during transfer on a reduced area of contact between transfer sheet and xerographic element, with consequent increase of charge density in the area of transfer and the prevalence of suflicient electrical potential difference at the transfer area 23 to cause effective transfer of the powder toner image under conditions of high relative humidity. The techniques disclosed are simple to carry out and are very effective.

With the procedures described, effective transfer has been practiced with the transfer member and apparatus in equilibrium with air at F. and 85 percent relative humidity. It will be observed that with both procedures described a transfer charge is established in successive limited areas of the transfer sheet material as it moves in contact with an image bearing surface so that transfer of successive portions of the image is alfected electrostatically, and that the successive limited areas of the transfer sheet material are rolled away from contact with the image bearing surface immediately after transfer thereto occurs.

While specific features of the invention have been disclosed variations in technique within the'scope of the appended claim are possible and are contemplated. There is no intention therefore of limitation to the exact details disclosed.

What is claimed is:

In the process .of xerography wherein an electrostati- CallY charged, 11%; xerographic plate comprising a photoconductive layer on a conductive backing is exposed to a light image to form an electrostatic latent image on the plate surface corresponding to the light image and the plate is developed with oppositely charged particles of developing material to form a xerographic powder image on the plate surface corresponding to the light image and the powder image is transferred to a transfer sheet, the improvement for effecting said powder image transfer under conditions of high relative humidity from the Xerographic plate to a flexible transfer sheet having a leading edge and a trailing edge and having the property of high lateral conductivity under conditions of high relative humidity comprising the steps of establishing surface contact between the leading edge of the transfer sheet and one end of the plate and simultaneously withholding the trailing edge of the transfer sheet from the remainder of the plate surface, whereby a limited area of the transfer sheet immediately adjacent the leading edge thereof is in contact with the plate, progressively contacting successive portions of the transfer sheet approaching its trailing edge with successive portions of the plate surface approaching its opposite end and simultaneously withdrawing the leading edge and successive portions of the sheet from the corresponding portions of the plate surface, whereby successive limited areas of the sheet are progressively placed in surface contact with the plate throughout the movement of the sheet, and maintaining a potential difference between the transfer sheet and the plate by forming an electrical circuit between an electrode attached only to the leading edge of the transfer sheet, a D. C. voltage source and the conductive backing of the xerographic plate, thereby to effect powder image transfer in such successive limited areas of contact.

References Cited in the file of this patent UNITED STATES PATENTS 2,558,900 Hooper July 3, 1951 2,576,047 Schafiert Nov. 20, 1951 2,684,902 Mayo et al. July 27, 1954 2,756,676 Steinhilper July 31, 1956 OTHER REFERENCES Non Destructive Testing, vol. 10, #1, Summer Numher 1951, pages 8 to 11. 

